1. Field of the Invention
The present disclosure relates to a device and a method for evaluating a cement bond. More specifically, the present disclosure relates to a device and method for evaluating the thickness, quality, and presence of a cement bond. Yet more specifically, the present disclosure concerns a device and method for evaluating a cement bond between cement and formation.
2. Description of Related Art
Hydrocarbon producing wellbores typically comprise casing set within the wellbore, where the casing is bonded to the wellbore by adding cement within the annulus formed between the outer diameter of the casing and the inner diameter of the wellbore. The cement bond not only adheres the casing 8 within the wellbore, but also serves to isolate adjacent zones within the formation from one another. Isolating adjacent zones can be important when one of the zones contains oil or gas and the other zone includes a non-hydrocarbon fluid such as water. Should the cement surrounding the casing be defective and fail to provide isolation of the adjacent zones, water or other undesirable fluid can migrate into the hydrocarbon producing zone thus diluting or contaminating the hydrocarbons within the producing zone.
The cement bond integrity can be assessed by a cement bond evaluation, which may be done by analyzing the attenuation characteristics of acoustic waves along the casing. The quality of the bond between cement and formation rock, however, can be difficult to assess due to the lack of means to quantify the cement layer parameters. If the thickness of the cement layer can be estimated and the coupling of the cement to the formation rock can be assessed, then they can be used to provide important parameters for the cement bond evaluation. For example, if the cement thickness adjacent a section of casing is found to be significantly different from that on an azimuthally opposed section, then the casing may not be centered in the borehole, indicating a poorly performed cementing job.
Downhole tools have been developed for analyzing the integrity of the cement bonding the casing to the wellbore. These downhole tools are typically lowered into the wellbore by wireline in combination with a pulley. The tools may include transducers disposed on their outer surface formed to be acoustically coupled to fluid in the borehole. These transducers are generally capable of emitting acoustic waves into the casing and recording the response of the acoustic waves as they travel, or propagate, across the surface of the casing.
However, the acoustic energy penetration through casing into the cement layer of some of these prior art acoustic techniques is limited by the high impedance contrast between casing and borehole fluid. In some of these devices, the acoustic waves are directed radially or at an angle to the casing surface and thus the highly rigid casing impairs the energy transmission into the casing. Additionally, it may be difficult to characterize the reflection signals arising from the inner and outer walls of the cement using prior art devices. The interference of currently used wave modes seriously impairs the recognition of the reflection effect; the interference of the wave modes and their reflections occurs when the wavelength in the cement layer becomes comparable with or exceeds cement layer thickness, which is a common scenario in oil field conditions. Moreover, these waves are also highly dispersive in the frequency range of the reflection measurement, making it difficult to characterize the reflection signals when the interference occurs. Nuclear logging techniques using gamma-ray penetration through casing have also been proposed for assessing bond integrity.